The high quality glasses of the present invention produce high optical quality chalcogenide fibers. Chalcogenide glasses are comprised of at least one chalcogen element (S, Se or Te) and other elements including, but not limited to, Ge, As, Ga, Sn, Sb, and transmit infrared light from between about 1 μm to about 12 μm or greater, depending on composition.
The infrared transmitting chalcogenide glasses and optical fibers encompass the IR region of interest with numerous applications including thermal imaging, temperature monitoring, and medical applications. Also, the chalcogenide glass fibers may be developed for IR missile warning systems and laser threat warning systems to provide superior aircraft survivability, and high energy IR power delivery using for example, but not limited to, CO (5.4 μm) and CO2 (10.6 μm) lasers.
In addition, these fibers may be developed for remote fiber optic chemical sensor systems for military facility clean up and other industrial applications. Chalcogenide glasses may also be used as bulk optical elements, including windows, lenses, prisms, beam splitters and the like, and must be the highest compositional uniformity and homogeneity in order to maintain accurate control of light rays passing through the glass and to achieve satisfactory optical results.
The chalcogenide glasses and fibers described herein, and more specifically arsenic sulfide based glasses and fibers, are developed for use in many defense applications including high energy IR laser power delivery for infrared countermeasures and defense facility clean up. High quality infrared transmitting optical fibers enable application in remote chemical sensors to detect contaminants in groundwater, environmental pollution monitoring, other civil/industrial process monitoring applications as well as Raman amplifiers and all optical ultra-fast switches for telecommunications, and fiber sources in the infrared for sensors. In addition, IR fibers are needed for biomedical surgery and tissue diagnostics.
To date, the prior art method to synthesize a chalcogenide glass from a melt is to heat the elemental precursors in an evacuated and sealed silica (quartz) glass ampoule and is demonstrated here by example.
Prior Art Process to Make Arsenic Sulfide-Based Glasses
First, arsenic and sulfur precursors sufficient to constitute a glass with the composition of 39% at. As and 61% at. S (71.88 grams and 48.12 grams respectively for a total of 120 grams) were loaded in a silica ampoule under an inert (e.g. Ar or nitrogen gas) atmosphere. The ampoule was connected to a vacuum pump and evacuated for 4 hours at 1×10−5 Torr. The ampoule was sealed using a methane/oxygen torch and placed inside a rocking furnace with a ±45° angle of inclination (FIG. 1) where it was heated and rocked according to a glass melting schedule, an example of which is shown for As39S61 glass in Table 1.
In Step 1, the top and bottom zones of the furnace were heated at a rate of 3° C./min from 20° C. (room temperature) to 750° C. The furnace then remained at 750° C. for 10 hours and was actively rocked at an inclination angle of ±45° to facilitate mixing and homogenization of the elemental components.
In Step 2, the furnace motion was stopped and the furnace was set to a vertical position (90° fixed angle) and held at temperature (750° C.) for 1 hour to facilitate fining and settling of the glass melt.
In Step 3, the temperatures of both zones were reduced at a rate of 5° C./min to 440° C. and the temperature was held at 440° C. for 2 hrs.
In Step 4, the hot ampoule was removed from the furnace and submerged in a room temperature water bath for 30 seconds to quench the glass, and was then placed in another furnace at 180° C. for 10 hours to anneal the solid glass.
TABLE 1Example of a prior art glass melting schedule forAs39S61 glass composition in a two-zone furnace.Heating RateTemperature (° C.)Temperature (° C.)DwellStep(° C./min)Top ZoneBottom Zone(Hours)Furnace Position1375075010Rocking at ± 45°inclination2—7507501Vertical 90° fixed354404402Vertical 90° fixed4Water quench
In Step 3 of the prior art process, although the top and bottom zones of the furnace are both set at the same temperature (440° C. in the example) the actual measured temperature along the length of the ampoule containing the glass melt may vary. A temperature gradient (ΔT) of 12° C. has been measured in the example (shown schematically in FIG. 2) and is due largely to convection heat loss through the top of the furnace. The effect of convection heat loss causes thermal convection currents within the bulk glass (shown as dashed curves in FIG. 3A), resulting in the condensation of glass beads above the melt at the cooler section of the ampoule which then drip back into the melt (shown in FIG. 3A & FIG. 3B).
These condensation beads may have a different composition than the rest of the glass melt and this continual mass fluxing cycle can cause a compositional non-uniformity throughout the entire melt.
Furthermore, as the glass cools during Step 3, the composition of the glass near the surface is changing as condensation of gaseous components (e.g. sulfur) from the closed system settle on the surface of the glass melt. Thermal convection currents within the glass are present during cooling and allow this surface glass, with a slightly different composition, to become reincorporated into the bulk glass.
The convection currents or swirls are not sufficient to thoroughly distribute or homogenize the glass, resulting in compositional gradients within the glass.
During water quenching of Step 4, the viscosity of the glass increases as the glass melt cools and the compositional gradients become frozen resulting in striae in the bulk glass.
Consequently, there are refractive index perturbations in the striae-containing glass that degrade the quality of the glass and fiber made from this glass. FIG. 4 shows an IR-image of a human hand and fingers viewed through a 1 inch diameter, 2.5 inches thick disk (both faces polished) of As39S61 glass of this example that was prepared using the method of the prior art, and reveals the presence of striae and refractive index perturbations within the glass.
The invention disclosed herein solves these long-standing problems and results in striae-free chalcogenide glasses with uniform refractive index.